It's based on a story from Bede's Ecclesiastical History of the English People. It comes from his obituary notice of Gregory the Great:

Nor is the account of St. Gregory, which has been handed down to us by the tradition of our ancestors, to be passed by in silence, in relation to his motives for taking such interest in the salvation of our nation. It is reported, that some merchants, having just arrived at Rome on a certain day, exposed many things for sale in the marketplace, and abundance of people resorted thither to buy: Gregory himself went with the rest, and, among other things, some boys were set to sale, their bodies white, their countenances beautiful, and their hair very fine. Having viewed them, he asked, as is said, from what country or nation they were brought? and was told, from the island of Britain, whose inhabitants were of such personal appearance. He again inquired whether those islanders were Christians, or still involved in the errors of paganism? and was informed that they were pagans. Then fetching a deep sigh from the bottom of his heart, "Alas! what pity," said he, "that the author of darkness is possessed of men of such fair countenances; and that being remarkable for such graceful aspects, their minds should be void of inward grace." He therefore again asked, what was the name of that nation? and was answered, that they were called Angles. "Right," said he, for they have an Angelic face, and it becomes such to be co-heirs with the Angels in heaven. What is the name," proceeded he, "of the province from which they are brought?" It was replied, that the natives of that province were called Deiri. "Truly are they De ira," said he, "withdrawn from wrath, and called to the mercy of Christ. How is the king of that province called?" They told him his name was Ælla: and he, alluding to the name said, "Hallelujah, the praise of God the Creator must be sung in those parts."

http://www.fordham.edu/halsall/basis/bede-book2.html

Gregory died in 605. Bede was writing 120-130 years later. It was in this very work that Bede popularised the use of BC and AD for dating purposes.

Bingley

Bingley